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首页> 外文期刊>Proceedings of the National Academy of Sciences of the United States of America >Kinetics and mechanistic analysis of an extremely rapid carbon dioxide fixation reaction
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Kinetics and mechanistic analysis of an extremely rapid carbon dioxide fixation reaction

机译：极快的二氧化碳固定反应的动力学和机理分析

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摘要

Carbon dioxide may react with free or metal-bound hydroxide to afford products containing bicarbonate or carbonate, often captured as ligands bridging two or three metal sites. We report the kinetics and probable mechanism of an extremely rapid fixation reaction mediated by a planar nickel complex [Ni~(Ⅱ)(NNN)(OH)]~(1-) containing a tridentate 2,6-pyridinedicarboxamidate pincer ligand and a terminal hydroxide ligand. The minimal generalized reaction, is M-OH + CO_2 → M-OCO_2H; with variant M, previous rate constants are (<)10~3 M~(-1) s~(-1) in aqueous solution. For the present bimolecular reaction, the (extrapolated) rate constant is 9.5 × 10~5 M~(-1) s~(-1) in N, N'-dimethylformamide at 298 K, a value within the range of k_(cat)/K_M≈10~5-10~8 M~(-1) s~(-1) for carbonic anhydrase, the most efficient catalyst of CO_2 fixation reactions. The enthalpy profile of the fixation reaction was calculated by density functional theory. The initial event is the formation of a weak precursor complex between the Ni-OH group and CO_2. followed by insertion of a CO_2 oxygen atom into the Ni-OH bond to generate a four center Ni(η~2-OCO_2H) transition state similar to that at the zinc site in carbonic anhydrase. Thereafter, the Ni-OH bond detaches to afford the Ni(η~1-OCO_2H) fragment, after which the molecule passes through a second, lower energy transition state as the bicarbonate ligand rearranges to a conformation very similar to that in the crystalline product. Theoretical values of metric parameters and activation enthalpy are in good agreement with experimental values [△H~(++) = 3.2(5) kcal/mol].
机译：二氧化碳可以与游离的或与金属结合的氢氧化物反应，得到含有碳酸氢盐或碳酸盐的产物，通常以桥接两个或三个金属位点的配体形式捕获。我们报告了由平面镍络合物[Ni〜（Ⅱ）（NNN）（OH）]〜（1-）介导的极快速固定反应的动力学和可能的机理，该络合物包含三齿2,6-吡啶二甲酰胺基钳型配体和一个末端氢氧化物配体。最小的广义反应是M-OH + CO_2→M-OCO_2H;对于变体M，先前的速率常数为水溶液中的（<）10〜3 M〜（-1）s〜（-1）。对于当前的双分子反应，在298 K的N，N'-二甲基甲酰胺中，（外推）速率常数为9.5×10〜5 M〜（-1）s〜（-1），该值在k_（cat ）/K_M≈10〜5-10〜8 M〜（-1）s〜（-1）用于碳酸酐酶，这是最有效的CO_2固定反应催化剂。通过密度泛函理论计算固定反应的焓分布。最初的事件是在Ni-OH基团和CO_2之间形成弱的前体复合物。然后将CO_2氧原子插入Ni-OH键中，以产生类似于碳酸酐酶中锌位的四中心Ni（η〜2-OCO_2H）过渡态。此后，Ni-OH键分离以提供Ni（η〜1-OCO_2H）片段，此后分子通过第二个较低的能量跃迁状态，因为碳酸氢盐配体重新排列成与晶体产物非常相似的构象。度量参数和活化焓的理论值与实验值[△H〜（++）= 3.2（5）kcal / mol]吻合良好。

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